Tomographic scan

ABSTRACT

A system for performing a scanning session of a patient&#39;s body, the system comprising: a radiation source; a radiation sensor; a patient positioner for placing the patient between the radiation source and the radiation sensor in a fixed position during each scan; at least one Pseudo-Random Grid (PRG) comprising randomly spaced perpendicular gridlines, the PRG being at least partially radiopaque and positioned between the radiation source and the radiation sensor in a fixed manner with respect to the patient positioner during the scanning session, wherein at least one of said radiation source and said radiation sensor is configured to move independently, and wherein at least one of said radiation source and said radiation sensor is moved to multiple different positions during the scanning session such that energy emitted from the radiation source passes through said PRG and said patient&#39;s body and collected by a surface of said radiation sensor.

RELATED APPLICATION

This application claims the benefit of priority under 35 USC § 119(e) ofU.S. Provisional Patent Application No. 62/114,906 filed 11 Feb. 2015,the contents of which are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

The invention relates to the field of tomography.

BACKGROUND

Tomography may refer to imaging by sections or sectioning, through theuse of any kind of penetrating wave. Tomography may be used inradiology, archaeology, biology, atmospheric science, geophysics,oceanography, plasma physics, materials science, astrophysics, quantuminformation, and other sciences. In most cases it is based on themathematical procedure called tomographic reconstruction.

Tomosynthesis is a method for performing high-resolution limited-angletomography. Tomosynthesis dates back to the 1930s, where film-basedradiography systems were used, with long exposure from a moving X-raytube. Today, tomosynthesis combines digital image capturing andprocessing with simple tube/detector motion as used in conventionalcomputerized tomography (CT). Although there are some similarities toCT, it is a separate technique. In CT, the source and detector make atleast a 180-degree rotation about the subject, obtaining a complete setof data from which images may be reconstructed. In digitaltomosynthesis, only a limited rotation angle (e.g., 15-60 degrees) maybe used, with a relatively small number of discrete exposures (typically10-80). This incomplete set of projections may be digitally processed toyield images, similar to conventional tomography but with a limiteddepth of field. However, because the image processing is digital, aseries of slices at different depths and with different thicknesses maybe reconstructed from the same acquisition, saving both time andradiation dose.

CT scans are typically performed by dedicated, high-end systems, withfast continuous rotations and with high-accuracy encoders for exactpositioning of the system components. Tomosynthesis, on the other hand,is traditionally performed by simpler, less costly system, withcompromised mechanical accuracy and robustness. Therefore, radiopaqueobjects (“fiducials”) are commonly used in the tomosynthesis scans forbetter mutual alignment of the various views. Fiducial are generallyfully opaque objects and therefore their trace may not be removed fromthe resulting image.

In the most general system, where both the detector and the X-ray sourcemove freely, there are nine degrees-of-freedom (DOFs): three for theposition of the detector center, three angular DOFs for the detectororientation in space and three for the X-ray source. Typically, a lessgeneral geometry is used, with up to eight DOFs, while one DOF is beingheld fixed. For example, the distance between the source and thedetector might be kept constant. For that purpose, at least fourfiducial points have to be used, each contributing two independentmeasures of x- and y-coordinates. However, more fiducials are typicallyused.

U.S. Pat. No. 5,359,637 to Webber discloses a self-calibratingtomosynthetic x-ray system. A calibrated device for recordingradiographic images of a selected object irradiated by a source ofradiation includes a first radiolucent radiographic recording medium inthe form of a CCD device for recording a first projected radiographicimage of the selected object. A second radiographic recording medium inthe form of a CCD device is supported in fixed generally parallelposition relative to the first radiographic recording medium to permitradiation from the source to pass through the first radiographicrecording medium and to impinge upon the second radiographic recordingmedium for recording a second projected radiographic image of theselected object. A radiopaque fiducial reference in the form of a gridis supported in fixed position generally between the first and secondradiographic recording mediums to permit a projected image of theradiopaque fiducial reference to be recorded on the second radiographicrecording medium. Projected radiographic images of the object and thefiducial reference are then recorded at different arbitrary relativepositions between the source of radiation and the object, the fiducialreference, and the recording mediums. An image of a selected object at aselected slice position through the object is synthesized from selectedprojected radiographic images of the object and the fiducial referencerecorded by the calibrated device.

U.S. Pat. No. 6,888,924 to Claus et al. discloses geometry of atomosynthesis system including a detector and an x-ray source, which isdetermined using fiducial markers with non-determined positions. Thegeometry is determined by arbitrarily identifying at least two markerswithin an imaged volume, at different relative distances between thedetector and the x-ray source, without having projections located on astraight line for all different source positions, and locating theprojections of the markers within at least two images acquired of theimaged volume. The at least two images correspond to different positionsof a focal spot of the x-ray source.

US Patent Application Publication No. 2012/0014498 to Akahori disclosesa radiographic imaging apparatus which includes: a radiation source forapplying radiation to a subject and at least one marker; a detectingunit for detecting the radiation transmitted through the subject; and animage obtaining unit for moving the radiation source relative to thedetecting means, applying the radiation to the subject from a pluralityof radiation source positions provided by the movement of the radiationsource, and obtaining a plurality of images corresponding respectivelyto the radiation source positions. The apparatus further includes aradiation source position obtaining unit for obtaining positionalinformation of each radiation source position of interest relative to areference radiation source position among the radiation source positionsbased on at least one marker image contained in each of a referenceimage obtained with the reference radiation source position and an imageof interest obtained with the radiation source position of interest.

The foregoing examples of the related art and limitations relatedtherewith are intended to be illustrative and not exclusive. Otherlimitations of the related art will become apparent to those of skill inthe art upon a reading of the specification and a study of the figures.

SUMMARY

The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools and methods which aremeant to be exemplary and illustrative, not limiting in scope.

There is provided, in accordance with an embodiment, a system forperforming a scanning session of at least a portion of the body of apatient, the system comprising: a radiation source; a radiation sensor;a patient positioner for placing the patient between the radiationsource and the radiation sensor in a fixed position during each scan ofthe scanning session; at least one pseudo-random grid comprisingrandomly spaced perpendicular gridlines, the pseudo-random grid being atleast partially radiopaque and positioned between the radiation sourceand the radiation sensor in a fixed manner with respect to the patientpositioner during the scanning session, wherein at least one of saidradiation source and said radiation sensor is configured to moveindependently, and wherein at least one of said radiation source andsaid radiation sensor is moved to multiple different positions duringthe scanning session such that energy emitted from the radiation sourcepasses at least partially through said pseudo-random grid and said atleast a portion of the body of the patient and collected by a surface ofsaid radiation sensor.

There is provided, in accordance with another embodiment, an add-on toan imaging system comprising: at least one pseudo-random grid comprisingperpendicular gridlines which are randomly spaced, the pseudo-randomgrid being at least partially radiopaque; and a mount for coupling thepseudo-random grid with the imaging system.

There is provided, in accordance with a further embodiment, a method forperforming a scan session of at least a portion of the body of apatient, the method comprising: positioning at least one pseudo-randomgrid, at least partially radioopaque, in a fixed position with respectto a position of the patient, wherein the pseudo-random grid comprisesperpendicular gridlines which are randomly spaced, multiply positioningat least one of a radiation source and a radiation sensor during thescanning session, wherein a corresponding at least one of the radiationsource and the radiation sensor is configured to move independently, toobtain multiple scans of said at least portion of the body of saidpatient, wherein said at least one pseudo-random grid is at leastpartially imaged in said multiple scans; and obtaining said multiplescans of said at least portion of the body of said patient.

There is provided, in accordance with yet another embodiment, a methodfor evaluating the geometry of a projection of a fiducial marker in animage, wherein the fiducial marker is an alignment grid comprisingperpendicular gridlines, the alignment grid being at least partiallyradiopaque, the method comprising using at least one hardware processorfor: identifying straight lines in the image; identifying a first set ofparallel lines of said straight lines by iteratively determining whichof said straight lines converge to the same point, wherein said firstset of parallel lines represents a first group of parallel gridlines ofsaid perpendicular gridlines; identifying a second set of parallel linesfrom the remainder of said straight lines by iteratively determiningwhich of said remainder of said straight lines approximately converge tothe same point, wherein said second set of parallel lines represents asecond group of parallel gridlines of said perpendicular gridlines whichare perpendicular to the first group of parallel gridlines; calculatingfeature points by calculating intersection points between the first setof parallel lines and the second set of parallel lines; and evaluating ahomographic matrix for transforming the space of the image to areference frame of the fiducial marker by correlating the feature pointsto the intersection points of the perpendicular gridlines of thefiducial marker.

There is provided, in accordance with yet a further embodiment, acomputer program product for evaluating the geometry of a projection ofa fiducial marker in an image, wherein the fiducial marker is analignment grid comprising perpendicular gridlines, the alignment gridbeing at least partially radiopaque, the computer program productcomprising a non-transitory computer-readable storage medium havingprogram code embodied therewith, the program code executable by at leastone hardware processor to: identify straight lines in the image;identify a first set of parallel lines of said straight lines byiteratively determining which of said straight lines approximatelyconverge to the same point, wherein said first set of parallel linesrepresents a first group of parallel gridlines of said perpendiculargridlines; identify a second set of parallel lines from the remainder ofsaid straight lines by iteratively determining which of said remainderof said straight lines approximately converge to the same point, whereinsaid second set of parallel lines represents a second group of parallelgridlines of said perpendicular gridlines which are perpendicular to thefirst group of parallel gridlines; calculate feature points bycalculating intersection points between the first set of parallel linesand the second set of parallel lines; and evaluate a homographic matrixfor transforming the space of the image to a reference frame of thefiducial marker by correlating the feature points to the intersectionpoints of the perpendicular gridlines of the fiducial marker.

In some embodiments, the pseudo-random grid is tilted with respect tothe surface of said radiation sensor.

In some embodiments, the system has nine degrees of freedom comprisingthree translational degrees of freedom of said radiation sensor, threerotational degrees of freedom of said radiation sensor and threetranslational degrees of freedom of said radiation source.

In some embodiments, one or more of said nine degrees of freedom is heldfixed.

In some embodiments, the one or more degrees of freedom which are heldfixed relate to constraints selected from the group consisting of: aconstant distance between the radiation source and the center of theradiation sensor, a tilt angle of the radiation sensor, confining themotion of the radiation sensor to a predefined plane, setting theprojection of the radiation source to coincide with the center of theradiation sensor and a motion of the radiation sensor being dependent onthe motion of the radiation source.

In some embodiments, the one or more degrees of freedom which are heldfixed relate to a motion of the radiation sensor being dependent on themotion of the radiation source, a guide point in space is selected, andthe motion of the radiation sensor is set to keep a constantmagnification of said guide point.

In some embodiments, the one or more degrees of freedom which are heldfixed relate to a motion of the radiation sensor being dependent on themotion of the radiation source, a guide point in space is selected, andthe radiation sensor is placed such that radiation emanating from saidradiation source and crossing the guide point hits a constant pixel ofthe radiation sensor.

In some embodiments, the at least one pseudo-random grid comprises twoor more pseudo-random grids, and wherein at least two of said two ormore pseudo-random grids are tilted one with respect to the other in oneor more predefined angles correspondingly.

In some embodiments, the method further comprises using at least onehardware processor for: evaluating the geometry of a projection of saidat least one Pseudo-random grid in each one of said multiple scans forregistering said multiple scans; and performing image reconstructionbased on said registering of said multiple scans.

In some embodiments, the evaluating of the geometry of the projection ofsaid at least one pseudo-random grid in each one of said multiple scanscomprises further using said at least one hardware processor withrespect to each scan of said multiple scans for: identifying straightlines in the scan; identifying a first set of parallel lines of saidstraight lines by iteratively determining which of said straight linesapproximately converge to the same point, wherein said first set ofparallel lines represents a first group of parallel gridlines of saidperpendicular gridlines; identifying a second set of parallel lines fromthe remainder of said straight lines by iteratively determining which ofsaid remainder of said straight lines approximately converge to the samepoint, wherein said second set of parallel lines represents a secondgroup of parallel gridlines of said perpendicular gridlines which areperpendicular to the first group of parallel gridlines; calculatingfeature points by calculating intersection points between the first setof parallel lines and the second set of parallel lines; and evaluating ahomographic matrix for transforming the space of the scan to a referenceframe of the at least one pseudo-random grid by correlating the featurepoints to the intersection points of gridlines of the at least onepseudo-random grid.

In some embodiments, the identification of a first set of parallel linesof said straight lines is performed by: performing for each two straightlines of the identified straight lines the following steps: a. computingthe convergence point of the two straight lines, b. determining each oneof the straight lines, as an inlier, if it approximately converges tosaid convergence point, or as an outlier, if it does not, to receive acurrent division of the straight lines into inliers and outliers, c.calculating the average convergence point of the inliers and determiningit to be the current convergence point, and d. iteratively dividing theinliers into current inliers and outliers based on the currentconvergence point and iteratively computing the current convergencepoint based on the current inliers to receive a final division of thestraight lines into inliers and outliers associated with said twostraight lines and with said current convergence point, and identifyingthe current convergence point associated with the final division havingthe least number of outliers, wherein the inliers of said final divisionhaving the least number of outliers are identified as the first set ofparallel lines of said straight lines, and wherein the identification ofsaid second set of parallel lines from the remainder of said straightlines is performed by: performing said steps a to d for each twostraight lines of the remainder of said straight lines, and identifyingthe current convergence point which is associated with the finaldivision having the least number of outliers, wherein the inliers ofsaid final division having the least number of outliers are identifiedas the second set of parallel lines.

In some embodiments, the at least one pseudo-random grid comprisesperpendicular gridlines which are randomly spaced.

In some embodiments, the identifying of the lines in the image isperformed by applying the Hough transform to the image.

In addition to the exemplary aspects and embodiments described above,further aspects and embodiments will become apparent by reference to thefigures and by study of the following detailed description.

BRIEF DESCRIPTION OF THE FIGURES

Exemplary embodiments are illustrated in referenced figures. Dimensionsof components and features shown in the figures are generally chosen forconvenience and clarity of presentation and are not necessarily shown toscale. The figures are listed below.

FIG. 1 shows an exemplary add-on to an imaging system for performing atomosynthesis scanning session of at least a portion of the body of apatient according to an embodiment;

FIG. 2 shows a pseudo-random grid of the exemplary add-on of FIG. 1;

FIG. 3 shows a flowchart of a method for performing a scan session of atleast a portion of the body of a patient, constructed and operative inaccordance with an embodiment of the disclosed technique; and

FIG. 4 shows a flowchart of a method for evaluating of the geometry ofthe projection of an alignment grid in a scan, constructed and operativein accordance with an embodiment of the disclosed technique.

DETAILED DESCRIPTION

The disclosed tomographic scan may produce three-dimensional (3D) imagesof body portions of a patient. The disclosed tomographic scan mayutilize one or more fiducial markers in the form of a grid (i.e., anetwork of perpendicular lines), which are termed herein below as“alignment grids”. These alignment grids may also be pseudo-random gridsby irregularly spacing their gridlines to receive a marker which may beuniquely identified even if only partially scanned.

The disclosed alignment grids may be an integral part of disclosedimaging systems or add-ons to existing imaging systems. A radiationsource and a radiation sensor (e.g. FPD) of the disclosed imagingsystems or of existing imaging systems, such as tomosynthesis imagingsystems, may be moved and rotated in relation to one another in anindependent manner and with up to nine degrees of freedom while takingmultiple images. The images (with the one or more alignment gridsshadows in them) may be then synthesized into 3D, resulting, forexample, in a semi-CT result, when X-ray imaging is concerned. Analgorithm for retrieving the trace of the disclosed alignment gridrelative to a reference frame in order to perform image registration isalso disclosed.

For example, tomosynthesis procedures are of low-dose in nature(typically, dose of tomosynthesis view is about 1% of the standardradiography dose), thus the noise level may be very high (e.g., tentimes higher). Since in the disclosed alignment grid all the radiopaquepoints are located along straight lines, the task of differentiatingbetween the grid points and the noise may be much easier. Hence, thereis no need to use the known strategies of using a large number of smallfiducial markers or large fiducial markers in order to overcome thenoise. These strategies may harden the task of differentiating andlocating of the markers in the image. Thus, using the disclosedalignment grids may improve the positioning accuracy, because opaquelines may be made very thin and still may be accurately evaluated. Asopposed to that, using many small fiducials may be disrupted bynoise-originated outliers and using large fiducials may not provide therequired geometrical resolution. Using a large number of fiducialmarkers in a noise-contaminated system may affect feasibility andreliability of the retrieving algorithm.

The term “scanning session”, as referred to herein, may relate to asession in which at least one scan is performed.

The terms “image” and “scan” and their derivations may be used hereinbelow in an interchangeable manner.

Reference is now made to FIG. 1, which shows an exemplary add-on to animaging system 100 for performing a tomosynthesis scanning session of atleast a portion of the body of a patient according to an embodiment.

In general, system 100 may be in accordance with medical imaging systemsas known in the art, such as X-ray or CT systems. In particular, system100 may be in accordance with prior art imaging systems, specificallyold prior art imaging systems such as an X-ray device, C-arm or U-arm ora full-rotation CT device.

System 100 may include a radiation source 110 and a radiation sensor120. System 100 may further include a patient positioner, such as bed160. Radiation source 110, radiation sensor 120, and bed 160 may or maynot be coupled with each other. System 100 may be operated by anoperator 170, which may be, for example, a technician or a care giver.An alignment grid 130 may be an integral part of system 100 or may becoupled with system 100 as an add-on, as shown in FIG. 1. The followingdescription will refer to alignment grid 130 as an add-on to system 100,where system 100 is an existing imaging system. However, the followingdescription also refers to an imaging system which includes thealignment grid as an integral part with the required modifications.

The add-on may further include a mount for coupling the imaging systemwith the alignment grid. Thus, system 100 may be coupled with alignmentgrid 130 via a mount 180. Mount 180 may mount alignment grid 130 to thepatient positioner, i.e., bed 160. Mount 180 may include two rigid orflexible stripes which may clip to alignment grid 130 at one end and tothe patient positioned at the other end. Mount 180 may be of otherforms, as known in the art, such as an arm. Alignment grid 130 may bedisposed between radiation source 110 and radiation sensor 120. Patient140 may be positioned between radiation source 110 and radiation sensor120. Alignment grid 130 may be located by mount 180 in a fixed positionwith respect to patient 140 and the patient positioner. This is in orderto allow registration of the different image views of patient 140received in the scan session based on alignment grid 130. Alignment grid130 may be located such that radiation emitted from radiation source 110towards the desired body portion of patient 140, at least partially passthrough it. For example, alignment grid 130 may be located betweenpatient 140 and radiation source 110, as shown in FIG. 1. Alternatively,alignment grid 130 may be located between patient 140 and radiationsensor 120. For example, as shown in FIG. 1, radiation source 110 may bepositioned above patient 140, radiation sensor 120 may be positionedbelow patient 140 and alignment grid 130 may be positioned betweenradiation source 110 and patient 140.

Radiation source 110 and/or radiation sensor 120 may be configured tomove independently during the scanning session. Accordingly, radiationsource 110 and/or radiation sensor 120 may be moved to multipledifferent positions, by that forming various dispositions of system 100,during the scanning session. Thus, a set of images may be formed,including traces of the patient and the alignment grid, as projectedthrough plurality of viewing angles and imaging magnifications.

System 100 may have nine degrees of freedom which include threetranslational degrees of freedom and three rotational degrees of freedomof radiation sensor 120, and three translational degrees of freedom ofradiation source 110. In some embodiments, one or more of the ninedegrees of freedom may be held fixed. This limitation may be achieved,for example, by keeping one degree of freedom degenerate or byrestricting the mutual radiation source-radiation sensor motions. Adegree of freedom which may be held fixed may relate to constraints suchas: a constant distance between radiation source 110 and the center ofradiation sensor 120, a tilt angle of radiation sensor 120 (i.e.,allowing only two rotational degrees of freedom), confining the motionof radiation sensor 120 to a predefined plane (i.e., allowing only twotranslational degrees of freedom), setting the projection of radiationsource 110 to coincide with the center of radiation sensor 120 and amotion of radiation sensor 120 being dependent on the motion ofradiation source 110.

If the degree of freedom which is held fixed relates to the constraintof a motion of radiation sensor 120 being dependent on the motion ofradiation source 110, a guide point in space may be selected. In someembodiments, the motion of radiation sensor 120 may be set to keep aconstant magnification of the guide point. In some other embodiments,radiation sensor 120 may be placed such that radiation emanating fromradiation source 110 and crossing the guide point hits a constant pixelof radiation sensor 120.

At least one of radiation source 110 and radiation sensor 120 may bemoved to multiple different positions during the scanning session inwhich energy emitted from radiation source 110, such as energy beams150A, 150B and 150C, pass at least partially through alignment grid 130and the desired body portion of patient 140 and collected by a surfaceof radiation sensor 120. As detailed herein above, alignment grid 130remains in a fixed position with respect to the position of patient 140during the entire scanning session.

Radiation source 110 may be, for example, an X-ray radiation source.Radiation sensor 120 may be a flat panel detector (FPD).

Alignment grid 130 may be typically a rectangular sheet of transparentmaterial, such as glass, plastic or Carbon sheet, having formed thereona grid (i.e., perpendicular gridlines), e.g., by printing or other formof material deposition. The gridlines of alignment grid 130 may be atleast partially radiopaque. The gridlines of alignment grid 130 may bemade of less-absorbing material, comparing to common fiducial markers,because fewer points may be required for the definition of a straightline. Accordingly, the gridlines may block between 5% and 30% of theradiation beam. In some embodiments, the gridlines may block between 5%and 10% of the radiation beam. In some embodiments, the gridlines mayblock between 10% and 20% of the radiation beam. In some embodiments,the gridlines may block between 20% and 30% of the radiation beam. Insome embodiments, the gridlines may block at least 5% of the radiation,at least 10% of the radiation, at least 20% of the radiation or at least30% of the radiation. Thus, the gridlines may not fully block theradiation impinging them. In addition, due to the partialradiopaqueness, one may still see what lies beneath the gridlines andthe gridlines may be more easily removed from the image to produce aclear image of a body portion of a patient.

Alignment grid 130 may be positioned such that its plane is parallel ortilted with respect to the surface of radiation sensor 120. Alignmentgrid 130 may be disposed between radiation source 110 and radiationsensor 120, such that the resulting scan shows the shadow of alignmentgrid 130. Since the relative location of patient 140 and alignment grid130 is fixed during the scanning session, one may use alignment grid 130to perform image registration to the scans received in each scanningsession.

In some embodiments, alignment grid 130 may be a Pseudo-Random Grid(PRG) which includes gridlines which are randomly spaced. In someembodiments, the set of distances between neighboring pairs of therandomly spaced gridlines may be a random set of different numbers.Therefore, when the RPG is projected upon radiation sensor 120, two setsof mutually-perpendicular converging lines are formed, each of which ispseudo-randomly spaced. Since the gridlines are randomly-spaced, the PRGposition may be evaluated even if it is only partially imaged. Referenceis now made to FIG. 2, which shows pseudo-random grid 130 of theexemplary add-on of FIG. 1. PRG 130 may include gridlines indicated 132and 134. Gridlines 132 may form a first set of parallel straight linesand gridlines 134 may form a second set of parallel straight lines,while gridlines 132 are perpendicular to gridlines 134 and vice versa.Gridlines 132 and 134 may be randomly spaced, i.e., the spaces betweengridlines 132 and the spaces between gridlines 134 are not constant. Insome embodiments, only gridlines 132 or gridlines 134 may be randomlyspaced. In some embodiments only a portion of gridlines 132 and/or 134may be randomly spaced. It should be noted that by referring herein to agrid as “randomly spaced” it is meant that at least a portion of one setof the two sets of parallel gridlines is randomly spaced.

In some embodiments, system 100 may include two or more alignment gridssuch as alignment grid 130, located in different positions with respectto one or more body portions of interest of patient 140. At least two ofthe two or more alignment grids may be tilted one with respect to theother in one or more predefined angles, correspondingly. Suchconfiguration may be advantageous, for example, when using an imagingsystem with nine DOFs.

The patient positioner may be configured to hold the patient in a fixedmanner during the scanning session. The patient positioner may be, forexample, bed 160, a chair or any other fixture for this purpose.

Using alignment grid 130 may enhance the imaging accuracy and may savethe need for relatively expensive encoders.

Reference is now made to FIG. 3, which shows a flowchart of a method forperforming a scan session of at least a portion of the body of apatient, constructed and operative in accordance with an embodiment ofthe disclosed technique.

In a step 200, at least one alignment grid, at least partiallyradiopaque, may be positioned in a fixed position with respect to aposition of the patient. The alignment grid may include a network ofperpendicular gridlines. The alignment grid may be similar to alignmentgrid 130 of system 100 of FIGS. 1 and 2. With reference to FIG. 1,alignment grid 130 may be positioned in a fixed manner above a portionof the body of patient 140 to be scanned.

In a step 210, a radiation source and/or a radiation sensor may bemultiply positioned during the scanning session. The radiation sourceand/or the radiation sensor may be configured to move independently, toobtain multiple scans of the at least portion of the body of thepatient. The at least one alignment grid may be at least partiallyimaged in the multiple scans. The radiation source and the radiationsensor may be similar to and may be moved in accordance with radiationsource 110 and radiation sensor 120 of system 100 of FIG. 1. Withreference to FIG. 1, radiation source 110 is positioned above a portionof the patient's body of interest. Radiation sensor 120 may bepositioned beneath the portion of the patient's body of interest.

In a step 220, multiple scans of the at least portion of the body of thepatient may be obtained. Such multiple scans may be obtained, forexample, by using system 100 of FIG. 1 and according to techniques knownin the art.

In an optional step 230, the geometry of a projection of the at leastone alignment grid in each one of the multiple scans may be evaluatedfor the purpose of registering the multiple scans. A method forevaluating the geometry of the projection of an alignment grid in a scanis disclosed below with respect to FIG. 4.

In an optional step 240, image reconstruction may be performed based onthe registration of the multiple scans. The image reconstruction may beperformed according to any of the prior art techniques such asshift-and-add, filtered back-projection or algebraic method.

Reference is now made to FIG. 4, which shows a flowchart of a method forevaluating the geometry of the projection of an alignment grid in ascan, constructed and operative in accordance with an embodiment of thedisclosed technique.

The method of FIG. 4 may be used to perform step 240 of the method ofFIG. 3 by performing the method of FIG. 4 with respect to each scan ofthe multiple scans. Alternatively, the method of FIG. 4 may be usedindependently for evaluating the geometry of a projection of a fiducialmarker in an image, where the fiducial marker is in the form of a gridand includes a network of perpendicular gridlines.

In a step 300, straight lines in the scan may be identified. Theidentifying of the lines in the scan may be performed by applying Houghtransform to the scan. One may then consider the transform-points whichare stronger than a certain predefined threshold.

However, few or more lines might be seen that are not related to thealignment grid. Further, the scan may be noisy, so few or more of thetrue gridlines of the alignment grid may be masked by noise. Therefore,the following steps are aimed at iteratively estimating the lines.

In a step 310, a first set of straight lines may be identified byiteratively determining which of the straight lines converge to the samepoint. The first set of lines may represent a first group of parallelgridlines of the perpendicular gridlines of the alignment grid.

The identification of the first set of parallel lines may be performedby performing the following steps for each two straight lines of theidentified straight lines.

In a first step, the convergence point of the two straight lines may becomputed.

In a second step, each one of the straight lines may be determined as aninlier, if it approximately (i.e., up to a predefined threshold)converges to the convergence point, or as an outlier, if it does not.Thus, a current division of the straight lines into inliers and outliersmay be received.

In a third step, the average convergence point of all the inliers (i.e.,an average of the convergence points of all the inliers) may becalculated and determined to be the current convergence point.

In a forth step, the inliers may be iteratively divided into currentinliers and outliers based on the current convergence point. The currentconvergence point may be iteratively computed based on the currentinliers. This iterative process may continue until convergence into theactual inliers is achieved. Thus, a final division of the straight linesinto inliers and outliers, associated with the two straight lines andthe current convergence point, may be received.

Then, the current convergence point which is associated with the finaldivision that has the least number of outliers may be identified. Theinliers of such final division may be identified as the first set ofparallel lines.

In a step 320, a second set of parallel lines from the remainder of thestraight lines may be identified by iteratively determining which of theremainder of the straight lines approximately converge to the same point(i.e., up to a predefined threshold). By “the remainder of the straightlines” it is meant the straight lines which are left after removing thefirst set of parallel lines from the identified straight lines. Thesecond set of parallel lines may represent a second group of parallelgridlines which are perpendicular to the first group of parallelgridlines. The identification of the second set of parallel lines fromthe remainder of the straight lines may be performed by repeating theabove steps detailed with respect to the identification of the first setof parallel lines. These steps may be performed for each two straightlines of the remainder of the straight lines. The current convergencepoint which is associated with the final division that has the leastnumber of outliers may be identified. The inliers of such a finaldivision may be identified as the second set of parallel lines.

In a step 330, feature points may be calculated by calculatingintersection points between the first set of parallel lines and thesecond set of parallel lines. This may be performed by running over allpairs of lines in a grid formed by the identified perpendicular firstand second sets of parallel lines and calculating all the cross-sectionpoints (i.e., the feature points). These intersection points may be usedas fiducials for image registration.

In a step 340, a homographic matrix (of 3*3 dimensions) for transformingthe space of the scan to a reference frame of the at least one alignmentgrid may be evaluated by correlating the feature points to theintersection points of gridlines of the at least one alignment grid. Ifthe alignment grid is a PRG, then the feature points may be uniquelycorrelated to their respective intersection gridline points.

System 100 of FIG. 1 may operate according to the method of FIG. 3.System 100 of FIG. 1 may further include at least one hardware processorconfigured to execute dedicated software in order to perform optionalsteps 240 and 250 of the method of FIG. 3 and/or perform the method ofFIG. 4. System 100 may then include a storage device for storing thededicated software.

It is preferable to remove the traces of the alignment grid (or of afiducial marker) from the scans before reconstructing the 3D image. Suchtask may be easier since the alignment grid, being comprised of straightlines, may be made of less-absorbing material. Hence, its projection(i.e., shadow) in the scans tends to be more faint. In addition,typically, one may utilize information of neighboring (uncovered) pixelsfor such task. A fiducial pixel might be surrounded by many otherfiducial pixels. On the other hand, a pixel of a gridline may typicallyhave six neighboring pixels that may be uncovered by the gridline.

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A system for performing a scanning session of atleast a portion of the body of a patient, the system comprising: aradiation source; a radiation sensor; a patient positioner for placingthe patient between the radiation source and the radiation sensor in afixed position during each scan of the scanning session; at least onepseudo-random grid comprising randomly spaced perpendicular gridlines,the pseudo-random grid being at least partially radiopaque andpositioned between the radiation source and the radiation sensor in afixed manner with respect to the patient positioner during the scanningsession, wherein at least one of said radiation source and saidradiation sensor is configured to move independently, and wherein atleast one of said radiation source and said radiation sensor is moved tomultiple different positions during the scanning session such thatenergy emitted from the radiation source passes at least partiallythrough said pseudo-random grid and said at least a portion of the bodyof the patient and collected by a surface of said radiation sensor; andwherein the system has nine degrees of freedom comprising threetranslational degrees of freedom of said radiation sensor, threerotational degrees of freedom of said radiation sensor and threetranslational degrees of freedom of said radiation source.
 2. The systemof claim 1, wherein the pseudo-random grid is tilted with respect to thesurface of said radiation sensor.
 3. The system of claim 1, wherein oneor more of said nine degrees of freedom is held fixed.
 4. The system ofclaim 3, wherein the one or more degrees of freedom which are held fixedrelates to constraints selected from the group consisting of: a constantdistance between the radiation source and the center of the radiationsensor, a tilt angle of the radiation sensor, confining the motion ofthe radiation sensor to a predefined plane, setting the projection ofthe radiation source to coincide with the center of the radiation sensorand a motion of the radiation sensor being dependent on the motion ofthe radiation source.
 5. The system of claim 3, wherein: the one or moredegrees of freedom which are held fixed relate to a motion of theradiation sensor being dependent on the motion of the radiation source,a guide point in space is selected, and the motion of the radiationsensor is set to keep a constant magnification of said guide point. 6.The system of claim 3, wherein: the one or more degrees of freedom whichare held fixed relate to a motion of the radiation sensor beingdependent on the motion of the radiation source, a guide point in spaceis selected, and the radiation sensor is placed such that radiationemanating from said radiation source and crossing the guide point hits aconstant pixel of the radiation sensor.
 7. The system of claim 1,wherein the at least one pseudo-random grid comprises two or morepseudo-random grids, and wherein at least two of said two or morepseudo-random grids are tilted one with respect to the other in one ormore predefined angles correspondingly.
 8. The system of claim 1,further comprising an add-on to the imaging system, the add-oncomprising a mount for coupling the pseudo-random grid with the imagingsystem.
 9. A method for performing a scanning session of at least aportion of the body of a patient, the method comprising: positioning atleast one pseudo-random grid, at least partially radio opaque, in afixed position with respect to a position of the patient, wherein thepseudo-random grid comprises perpendicular gridlines which are randomlyspaced, multiply positioning at least one of a radiation source and aradiation sensor during the scanning session, wherein a corresponding atleast one of the radiation source and the radiation sensor is configuredto move independently, to obtain multiple scans of said at least portionof the body of said patient, and wherein said at least one pseudo-randomgrid is at least partially imaged in said multiple scans; and obtainingsaid multiple scans of said at least portion of the body of saidpatient.
 10. The method of claim 9, wherein the method further comprisesusing at least one hardware processor for: evaluating the geometry of aprojection of said at least one pseudo-random grid in each one of saidmultiple scans for registering said multiple scans; and performing imagereconstruction based on said registering of said multiple scans.
 11. Themethod of claim 10, wherein the evaluating of the geometry of theprojection of said at least one pseudo-random grid in each one of saidmultiple scans comprises further using said at least one hardwareprocessor with respect to each scan of said multiple scans for:identifying straight lines in the scan; identifying a first set ofparallel lines of said straight lines by iteratively determining whichof said straight lines approximately converge to the same point, whereinsaid first set of parallel lines represents a first group of parallelgridlines of said perpendicular gridlines; identifying a second set ofparallel lines from the remainder of said straight lines by iterativelydetermining which of said remainder of said straight lines approximatelyconverge to the same point, wherein said second set of parallel linesrepresents a second group of parallel gridlines of said perpendiculargridlines which are perpendicular to the first group of parallelgridlines; calculating feature points by calculating intersection pointsbetween the first set of parallel lines and the second set of parallellines; and evaluating a homographic matrix for transforming the space ofthe scan to a reference frame of the at least one pseudo-random grid bycorrelating the feature points to the intersection points of gridlinesof the at least one pseudo-random grid.
 12. The method of claim 11,wherein the identification of a first set of parallel lines of saidstraight lines is performed by: performing for each two straight linesof the identified straight lines the following steps: a. computing theconvergence point of the two straight lines, b. determining each one ofthe straight lines, as an inlier, if it approximately converges to saidconvergence point, or as an outlier, if it does not, to receive acurrent division of the straight lines into inliers and outliers, c.calculating the average convergence point of the inliers and determiningit to be the current convergence point, and d. iteratively dividing theinliers into current inliers and outliers based on the currentconvergence point and iteratively computing the current convergencepoint based on the current inliers to receive a final division of thestraight lines into inliers and outliers associated with said twostraight lines and with said current convergence point, and identifyingthe current convergence point associated with the final division havingthe least number of outliers, wherein the inliers of said final divisionhaving the least number of outliers are identified as the first set ofparallel lines of said straight lines, and wherein the identification ofsaid second set of parallel lines from the remainder of said straightlines is performed by: performing said steps a to d for each twostraight lines of the remainder of said straight lines, and identifyingthe current convergence point which is associated with the finaldivision having the least number of outliers, wherein the inliers ofsaid final division having the least number of outliers are identifiedas the second set of parallel lines.
 13. The method of claim 10, whereinthe at least one pseudo-random grid comprises perpendicular gridlineswhich are randomly spaced.